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The antitumour effects of some ruthenium (II) complexes were tested in mice bearing the solid metastasizing tumour, Lewis lung carcinoma. The toxicity of trans-RuCl2 (DMSO)4 is 10-fold higher than that of its cis-isomer. Qualitatively the antitumour activity of these two complexes is comparable, although trans-isomer is more potent. Both exhibit only marginal effects on primary tumour growth while significantly lowering lung metastasis formation. The effects of trans-RuCl2 (DMSO)4 on primary tumour depend on the inoculum size, being more pronounced with low tumour inocula; significant antitumour effects can be achieved also by replacing Cl with I in the molecule. trans-RuCl2(DMSO)4 markedly reduces the number of lung metastases and particularly their volume; the reduction is comparable to that obtained with equitoxic treatments of cisplatin. Treatment with trans-RuCl2(DMSO)4 for 10 consecutive days, after surgical amputation of primary tumour, significantly prolongs the survival time of the treated mice; cisplatin, at equitoxic doses, is less effective. These data show that dimethylsulphoxide ruthenium(II) complexes possess a significant antitumour and antimetastatic activity in the Lewis lung system, also exhibiting an interesting therapeutic potential when combined with surgical amputation of the primary tumour.